Multi-chamber hollow cathode low voltage electron beam apparatus



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Q QWM United States Patent 3,411,035 MULTI-CHAMBER HOLLOW CATHODE LOW VOLTAGE ELECTRON BEAM APPARATUS William C. Necker, Cincinnati, and Charles I. McVey,

Shaker Heights, Ohio, assignors to General Electric Company, a corporation of New York Filed May 31, 1966, Ser. No. 554,007 13 Claims. (Cl. 315-111) ABSTRACT OF THE DISCLOSURE A hollow cathode structure having perforated or nonpeforated side walls is partitioned into two or more coupled chambers. The cathode is operable in a low pressure gaseous medium and at a relatively low cathode-toanode potential, the interaction of the gas and electric potential forming a body of ionized gas plasma in each cathode chamber. The plasmas are interconnected and an electron beam is emitted from one or more exit apertures in the cathode.

Our invention relates to certain improvements in electron beam irradiation apparatus of the gaseous beam type, and in particular, to improvement in the cathode structures of electron beam apparatus described in US. Patent 3,218,431 to L. H. Stauffer entitled Self-Focusing Electron Beam Apparatus, and in US. Patent 3,320,475 to K. L. Boring, entitled Nonthermionic I-Iollow Cathode Electron Beam Apparatus, both assigned to the assignee of the present invention.

The electron beam apparatus described in the abovementioned US. patents are especially useful in welding, heating and processing various materials in controlled gaseous environments. The apparatus comprises a housing within which is positioned a hollow, perforated or nonperforated (shielded) cathode structure adapted to be operable in a relatively low pressure ionizable gaseous medium and at a relatively high negative electrical potential relative to the housing, sufiicient to produce a body of ionized gas or plasma within the cathode. The cathode is a single chamber provided with an aperture through which an electron beam may issue from the plasma and pass to a work piece upon which it impinges to produce a desired effect. Within a particular range of gas pressure and cathode-to-housing potential, interaction between the gaseous medium and high potential generates a focussed or focusable electron beam having a beam current magnitude (beam intensity) determined primarily by the gas pressure and electrical potential.

While the hereinabove described electron beam apparatus are satisfactory for various applications, they require cathode operation at voltages higher than approximately kilovolts to obtain significant electron beam power densities for cathodes of diameters greater than approximately one inch. The significant power densities may also be obtained with similar but smaller dimension cathodes at voltages below 10 kilovolts, but at much lower total beam power levels. Thus, the generation of high (total) power electron beams with the cathodes described hereinabove introduces the hazards of attendant X-rays unless special shielding is provided to protect operating personnel. In addition, the high voltages present special problems in the design of the apparatus power supply and associated insulators to prevent corona discharge and areover.

3,411,035 Patented Nov. 12, 1968 "ice Therefore, one of the principal objects of our invention'is to' provide improved hollow cathode structures for generating electron beams of significant power level at relatively low cathode voltages and in relatively low pressure gaseous media.

Another object of our invention is to provide such cathode structures having multiple chambers whereby multiple bodies of plasma may be generated therein.

Briefly stated, our invention comprises an improved hol low cathode structure having perforated or nonperforated side walls, the cathode being divided into two or more coupled chambers. The cathode is arranged within a housing adapted to contain a low pressure ionizable gas and is electrically energized at a relatively low negative potential (generally below 10 kilovolts) relative to the housing such that in a desired mode of operation a plasma of ionized gas forms within each chamber of the cathode in coupled relationship, and a beam of electrons is emitted from the cathode through an exit aperture formed in a wall thereof. The cathode chambers and resultant plas mas therein may be series or series-parallel coupled to thereby obtain a relatively high power electron beam at low voltage operation and thus avoid the dangers of X-ray generation and the problems associated with high voltage corona discharge and arc-over.

The features of our invention which we desire to protect herein are pointed out with particularity in the .appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, wherein like parts in each of the several figures are identified by the same reference character, and wherein:

FIGURE 1 is an elevation view, partly in section, and partly schematic, illustrating an electron beam irradiation apparatus including a first embodiment of a multi-chamber cathode constructed in accordance with our invention;

FIGURE 2 illustrates typical operating curves for a particular FIGURE 1 cathode, indicating the variation of the magnitude of cathode current with cathode voltage for various pressures of a helium gaseous medium;

FIGURE 3 is a sectional side view of a second embodiment of our cathode structure;

FIGURE 4 is a sectional side view of a variation of the first and second embodiments of our cathode structure;

FIGURE 5 is a sectional side view of a second variation of the first and second embodiments of our cathode structure;

FIGURE 6 is a sectional side view of a third variation of the first and second embodiments of our cathode structure;

FIGURE 7 is a sectional side view of a third embodiment of our cathode structure;

FIGURE 8 is a sectional side view of a fourth embodiment of our cathode structure; and

FIGURE 9 (a and b) is a top and side view, partly in section, of a fifth embodiment of our cathode structure.

Referring particularly to the apparatus illustrated in FIGURE 1, there is shown a housing designated as a whole by numeral 1, having a cylindrical shape, although other forms may also be employed. Housing 1 comprises a top end plate 2 and hollow cylindrical (or other form) side wall 3 which may both comprise a single member, and bottom end plate 4. Housing 1 is constructed of a nonporous material, top end plate 2 and side wall 3 being preferably of an electrically nonconductive material, and

bottom end plate 3 being an electrically conductive material such as a suitable metal. Top end plate 2, if separate from side wall 3, is joined thereto by any known method appropriate to the material, the particular method not being critical since a high vacuum is not required within the housing. Bottom end plate 4, or a lower section of side wall 3, is made removable to facilitate the insertion and withdrawal of material 5 being electron beam processed in a container 6 which rests on bottom end plate 4. Container 6 may be made of copper or other suitable good electrically conductive and good heat conductive material and the anode of the apparatus is then considered as comprising bottom end plate 4 and container 6. Material 5 may, of course, be electrically conductive or nonconductive.

Our electron beam source or generator is an electron gun assembly consisting of a hollow perforated cathode structure 7, preferably in the form of a cylinder although other shapes may be employed, with an exit aperture 9 in the center of a wall thereof (bottom end wall being illustrated) wherefrom an electron beam is emitted by nontherm'ionic means in a manner described in detail in the aforementioned patent to Stauffer. The hollow cathode structure 7 is constructed from an electrically conductive material which is capable of being formed, has a relatively high melting point to avoid melting at the temperature to which the cathode may be subjected at high beam intensities even though no heat source as such is utilized, preferably does not emit significant amounts of gas at this temperature, and has relatively high secondary electron emission characteristics. A preferred embodiment of our perforated cathode for electron beam processing applications wherein temperatures in the order of 3,000 C. are required is a side wall construction from a perforated sheet or mesh of tantalum or molybdenum, although other surfaces characterized by a number of small openings therethrough are also appropriate as well as a nonperforated sheet, that is, a surface containing no other openings except for the exit aperture as illustrated in FIGURE 3. For lower temperature applications, the cathode may be constructed of stainless steel, copper or brass by way of further example. The top and bottom end walls of the cathode are preferably of a solid or nonperforated construction for added structural rigidity, or alternatively, may be perforated. The top and bottom end walls of the cathode are connected to the side wall thereof by any suitable method.

A power supply line connected to terminals 10 supplies electric power at an adjustably controlled direct current (DC) voltage to the cathode by means of cathode stem 12 and conductor 11 connected thereto for obtaining a relatively low negative cathode-to-anode DC. potential. The positive or ground side of the power supply line is connected to the anode at bottom end plate 4. Cathode stem 12 is electrically insulated from top end plate 2 by means of insulating bushing 13 which is compatible for the voltage range employed. Cathode stem 12 comprises a tubular, electrically conductive member which may be made of stainless steel for supporting and positioning cathode 7 within housing 1. The voltage supplied at terminals 10 is adjustable up to approximately 10 kilovolts (but may be considerably higher for high power applications) and may be provided by a conventional power supply consisting of an adjustable voltage alternating current source 15, whose voltage is increased by means of a step-up transformer 16 and converted to a filtered DC. voltage by means of rectifiers 17 and a conventional filter network which may include resistors, inductors and capacitors, and is illustrated as a whole by numeral 18.

A suitable ionizable gas, such as argon or helium, is introduced into the interior of the enclosure within housing 1 through passage means 19 which preferably passes through top end plate 2 or the upper portion of side wall 3. Passage means 19 is connected to a gas supply 20 through valve 21 for regulating the rate of gas flow into housing 1. A partitioning member (not shown) nonporous to the gaseous medium may be employed, if desired, to separate the housing into an upper enclosure containing the cathode and a lower enclosure containing the material 5 being irradiated and processed by an electron beam emitted from the cathode. This partitioning member would be provided with an aperture aligned with cathode aperture 9 and preferably be of size sufficient merely to permit passage of the electron beam therethrough and insufficient in size to permit substantial passage of any objectionable gases or vapors which may be generated by excessive gassing of a particular irradiated material 5 in the lower or processing enclosure. A second passage means 24, preferably located in a lower portion of side wall 3 provides, by virtue of its larger size, a low impedance exit for this generated gas, and thus aids in maintaining a desired gas pressure within housing 1, and is connected to a suitable exhaust pumping device 25 through throttle valve 26. Thus, possible contamination of the cathode by undesired gases generated by the irradiated material is largely prevented.

A theory for explaining the principle of electron beam formation and ejection from the hollow perforated cathode is described in the aforementioned Stautfer patent and will not be repeated herein. It is believed sufiicient to summarize such theory by describing the interior of the multiple cathode cavities in the present invention, while in the desired mode of operation, as comprising a glowing body of plasma or ionized gas (shown in dashed line) in each cavity. The plurality of bodies of plasma are interconnected and separated from the cathode walls by less luminous sheaths which are bounded by the walls. An external glow discharge or plasma surrounding the cathode in the case of the perforated cathode (and being directly below the cathode in the nonperforated cathode case) determines an ionized region of low voltage drop, separated from the cathode by a region of high voltage gradient or cathode dark space surrounding the aperture end of the cathode externally thereof. The interaction of the relatively low pressure gaseous medium and negative cathode-to-anode potential generates the bodies of plasma within particular ranges of gas pressure wherein for a particular cathode each gas pressure range is dependent upon the gas and voltage employed. A cathode diameter to cathode aperture ratio in the range of approximately 4:1 to 8:1 has been found satisfactory. In the perforated cathode structure, .the cathode side wall perforations may have a maximum size of approximately of the cathode aperture 9, and may be made as small as possible with the limiting case being the nonperforated structure with or without an external shield.

Referring now to the specific construction of the hollow perforated cathode 7 in FIGURE 1, which is shown in section, a partitioning member 27 separates the cathode cavity into two chambers. Member 27 may be constructed of the same material as the cathode side walls and is preferably made an integral part thereof. Member 27 has an aperture 28 which is centrally located and thus aligned with exit aperture 9, but this arrangement of apertures is not essential as will be described with reference to the FIGURE 9 embodiment. Apertures 9 and 28 need not be of the same dimension. An example of a typical cathode constructed in accordance with the FIG- URE 1 embodiment is as follows: The cathode is constructed of tantalum screening, .005 inch wire, 60 mesh, and is cylindrical in shape having a length of 1 /2 inches, diameter of 1% inches, apertures 9 and 28 are of 4 inch diameters and partitioning member 27 is located /2 inch from the exit aperture end of the cathode.

The operating characteristics of the specific cathode hereinabove described are illustrated in FIGURE 2 which illustrates a family of curves indicating the variation of the magnitude of cathode current in milliamperes (ma.) with cathode voltage (cathode-to-anode potential in kilovolts (kv.)) for cathode operation at various pressures of a helium gaseous medium. At the lower gas pressures (as depicted at 29 and 47 microns), a partially or fully developed body of plasma is formed within the lower chamber of the cathode. This single plasma body mode may include a pencil-shaped weak plasma in the upper cathode chamber. At the higher gas pressures (such as 88 microrfs) the single plasma body mode abruptly switches to the double plasma body mode (wherein both plasma bodies are fully developed) at a particular voltage determined by the gas and gas pressure. Thus, for a helium gas environment, and the particular cathode here= inabove described, the single plasma mode switches to the double mode at a cathode voltage of -3.3 kilovolts for a gas pressure of 88 microns.

The current, voltage and pressure relationships presented in FIGURE 2 for the specific cathode do not represent the total operating range of this cathode. Thus, this cathode is satisfactorily operable in the double plasma mode at a helium gas pressure range from ap proximately 70 to more than 300 microns. For the heavier gases, such as argon, the pressures are somewhat lower for the same beam power. It has been found that as the gas pressure is increased, the voltage at which the single double plasma mode transition occurs is decreased. Operation at higher values of total electron beam power is obtained by increasing the gas pressure (since the greater number of gas molecules produce a greater number of ions that form the plasma) and by increasing the cathodeto-anode potential. The total electron beam power is limited by the rating of the power supply in many cases, otherwise it is limited by excessive heating of the cathode by ion bombardment.

The cathode current-voltage characteristic in double plasma mode operation is considerably flatter than that obtained for single mode operation at similar beam power. This flatter characteristic contributes to a greater stability resulting in satisfactory beam mode operation over broader operating ranges of beam power, voltage and gas pressure as compared to the single chamber cathodes described in the patents to Staulfer and Boring.

The electron beam emitted from the plasma body in the lower cathode chamber and passing through exit aperture 9 to the material 5 may be focussed to a very small spot by employing a suitable electromagnetic or electrostatic lens, positioned internally or externally of housing 1. FIGURE 1 illustrates an electromagnetic lens 29 positioned internal of the housing and coaxial with the electron beam axis. The lens is connected to a source (not shown) of adjustably controlled D.C. voltage by means of insulated electrical conductors 30.

Beam shaping may be accomplished by employing aperture shapes which correspond to the desired electron beam outline in cross section. Thus, a beam circular in cross section is obtained by using a circular aperture whereas a rectangular beam results from using a rectangular shaped aperture.

It is to be noted that in the FIGURE 1 embodiment, the two cathode chambers or stages are serially coupled and the body of plasma within each cathode chamber contribute to the electron beam issuing through exit aperture 9. It is the combination of the higher gas pressures which are maintained in the vicinity of the cathode as compared to the lower gas pressures in the aforementioned cathode structures described in the Stauffer and Boring patents, and the series arrangement of the two cathode stages, which permits generation of a higher total power electron beam.

A second embodiment of our multi-stage hollow cathode is illustrated in FIGURE 3 wherein the hollow cathode 7 is a nonperforated structure having exit aperture 9 in the bottom end wall thereof wherefrom an electron beam may be emitted by nonthermionic means. The cathode may have the same shapes and be constructed of the same (but nonperforated) electrically conductive material as described with reference to the FIGURE 1 embodiment. The partitioning member 27 which separates the cathode cavity into two serially coupled chambers may be constructed of the same nonperforated material as the cathode walls, or may be of perforated construction. The additional feature illustrated in FIGURE 3, other than the use of a nonperforated structure 7, is an electrically conductive shield 32 made of sheet metal such as stainless steel, positioned in concentric relationship around cathode 7 and electrically insulated therefrom. Satisfactory beam mode operation of our multi-stage nonperforated cathode may be maintained, in the absence of shield 32, due to our use of lower voltages and higher gas pressures, as compared to the nonperforated cathode described in the aforementioned Bor= ing patent. Shield 32, if employed, is preferably of the same configuration as the cathode, has an open bottom end, and is operable at ground (anode) potential. Shield 32 is supported from top end plate 2 of housing 1 by a suitable means such as a metallic tubular member 33 provided with conventional clamping means 34 permitting axial adjustment of shield 32 with respect to cathode 7. A theory for explaining the operation of the nonperforated cathode is described in the Boring patent and will not be repeated herein. It will suffice to mention that our nonperforated cathode is operable in approximately the same ranges of voltage and gas pressure as our perforated cathode illustrated in FIGURE 1, and generates an electron beam of similar power magnitude.

A first variation of the multi-stage hollow cathode embodiment of FIGURE 3 (or FIIGURE 1) is illustrated in FIGURE 4 wherein the upper portion 7a of the cathode and the lower portion 7b thereof are spaced from each other, being separated by an electrically insulating member 40. Cathode portions 7a and 7b are each, in effect, separate single-chamber cathodes, and each may be of the nonperforated or perforated construction described with reference to FIGURES 3 and 1, respectively. Thus, the separate cathode chambers may be each of cylindrical, rectangular, square, or other shape, as desired. In like manner, the exit aperture 9, and aperture 28 may be of cylindrical, rectangular, or other shape. Insulating member 40 may comprise a suitable ceramic material appropriate to the particular temperatures anticipated as determined by the material 5 being electron beam irradiated. Insulating member 40 which is of annular construction, may have a thickness in the range of approximately to A3 inch. The bottom surface of upper cathode chamber 7a and the top surface of lower cathode chamber 7b are connected to insulating member 40 in any manner appropriate to the materials employed and the. cathode operating temperature.

A bias resistor 41 electrically interconnects the two cathode chambers to provide a selected bias potential between the two chambers during cathode operation. The two plasma bodies generated in the cathode chambers of the FIGURES 1 and 3 (and all the other) embodiments are separated by a potential difference of approximately 200 volts. Further, there is a potential dilference of approximately 400 volts between each plasma body and its associated cathode chamber walls. Plasma body to cathode chamber wall potential differences greater than approximately 400 volts can cause spark discharges. For this reason, it is desirable in many cases to isolate each plasma body and the bias resistor 41 is a first means for accomplishing this isolation. The bias resistor may have a value in the range of to 50,000 ohms resistance wherein the resistance is dependent upon the electron beam current and voltage, a higher ohm resistance being employed at low gas pressure levels which are generally associated with high voltage and low current for at particular beam power. The bias resistor is chosen to obtain a selected electrical potential difference between the two cathode chambers as determined by the particular operating conditions. The potential difference between the two cathode chambers is generally in the range between 50 to 500 volts and is determined primarily by the electrical potential difference inherently existing between the two bodies of plasma, this latter potential diiference accounting for the interconnection of the plasma bodies to maintain the energy balance thereof.

FIGURE illustrates a second variation of the FIG URES 1 and 3 cathode embodiments and is similar to the FIGURE 4 embodiment in that a means is provided for maintaining a selected potential difference between the two cathode chambers. The desired potential difference (in the range between 50 and 500 volts) is obtained by applying a bias voltage to lower cathode chamber 7b by means of an electrical conductor 50 connected to chamber 7b and supplied :from a source (not shown) of adjustably controlled DC voltage which may be separate from the power supply for upper cathode chamber 7a or may be an integral part thereof. The bias voltage supplied to lower chamber 7b is of a smaller magnitude than the voltage applied to upper chamber 7a. Thus, for example, if a voltage of -8,000 volts is supplied to chamber 7a via cathode stem 12, a bias voltage in the range of 7,500 to 7,950 volts is applied to lower chamber 7b via conductor 50.

FIGURE 6 illustrates a third variation of the FIG- URES 1 and 3 cathode embodiments wherein the lower cathode chamber 7b is completely isolated from upper chamber 7a as distinguished from the FIGURES 4 and 5 embodiments. During operation of our multi-stage cathode in the double plasma mode, the lower cathode chamber 7b assumes a potential difference relative to upper chamber 7a such that chamber 7b is maintained at the voltage of smaller magnitude as described hereinabove with reference to FIGURE 5. An additional fea ture of the FIGURE 6 embodiment is the extension of insulating member 40 (which separates and insulates the cathode chambers) into a ceramic sheath enveloping the multi-stage cathode. The ceramic sheath 40 is closely fitted to the chamber side walls, .and in the case of a perforated cathode structure, as illustrated, the ceramic sheath is constructed of a high secondary electron emission material such as magnesium oxide, to become a further source of electrons for the plasmas due to ion bombardment of the sheath. The ceramic sheath for either the perforated or nonperforated cathode improves the operating efiiciency thereof by minimizing the radiation heat loss from the plasma bodies. The ceramic sheath is char- 'acterized by having a thickness and porosity to be opaque to thermal radiation. The porosity may be as low as one percent and the thickness of the sheath is in the range of .020 inch to A inch.

A third embodiment of the multi-stage hollow cathode is illustrated in FIGURE 7 wherein the cathode partitioning member 27 is not perpendicular to the cathode side walls as illustrated in the first two embodiments, but instead, is disposed at an angle other than substantially 90 thereto. As illustrated, the cathode may be constructed entirely of the hereinabove described electrically conductive mesh or screen material, with a suitable con nection to cathode stem 12, or the top end wall thereof may be of a solid or nonperforated construction to provide added structural rigidity as shown in FIGURES 1 and 8. Alternatively, the entire cathode may be of nonperforated construction.

FIGURE 8 illustrates .a fourth embodiment of our cathode structure wherein the side walls are of frustoconical shape and two annular partitioning members 27a and 27b are respectively provided with apertures 28a and 28b to obtain a serially coupled three-stage cathode. It should be apparent that the first three cathode embodiments illustrated in FIGURES l, 3, 7, as well as the three variations of the FIGURES 1, 3 embodiments illustrated in FIGURES 4, 5, 6, may also be provided with one (or more) additional partitioning members to thereby obtain serially coupled three (or more) stage cathodes.

FIGURE 9 (a, 12) illustrates a fifth embodiment of the multi-stage hollow cathode wherein six cathode chambers are interconnected in a series-parallel arrangement. FIG- URE 9a is a. top view, and FIGURE 9b a sectional side view of six chambers each being of cylindrical shape. A' central or inner cathode chamber is surrounded along its cylindrical surface by four outer cathode chambers 91, 92, 93 and 94 which for illustrative purposes only are each indicated as being symmetrically arranged around chamber 90 in tangential relationship thereto and also tangential with each adjacent outer chamber. The five upper chambers 90 through 94 are coupled in parallel relationship since they are all connected to the power supply (not shown) via common cathode stem 12. Each of the upper five chambers is provided with only one aperture and these apertures 28a, 28b, 28c, 28d and 28e are disposed in the bottom end walls of the respective chambers, aperture 28a being centrally located and apertures 28b, 28c, 28d and 282 being located off-center. The five upper chambers 90-94 are connected to support plate 95 which is connected to cathode stem 12 to provide a rigid structure and a common feed from the power supply. These chambers may also be rigidly connected along the tangential portion of their side walls by any suitable means to provide additional structural rigidity to the cathode.

A sixth cathode chamber 96, provided with beam exit apertures 9 and having apertures 28a through 282 in the common (top) end wall thereof, is located below chambers 90 through 94 and is thus serially coupled to the upper five parallel stages. The body of plasma generated in each of chambers 90-94 is at the same potential relative to ground due to the common connection to the power supply and lack of interconnection of these five plasma, but the plasma in lower chamber 96 is at some lower potential as described hereinabove with reference to the FIGURE 4 embodiment. The plasma in each of the five parallel upper chambers are thus series connected with the plasma in the lower chamber to thereby provide the series-parallel arrangement. It should be understood that beam exit aperture 9 need not be aligned with aperture 28a since in each of our described multi-stage cathode embodiments it is the potential (difference in voltages) between adjacent plasma bodies which causes the interconnection thereof through a suitable aperture in the partitioning member rather than an axial alignment of the various cathode apertures. The electron beam is emitted from the cathode in all of the herein described embodiments through beam exit aperture 9 in a direction normal to the bottom surface of the cathode regardless of the orientation of the one or more apertures 28 within the cathode partitioning members.

An alternate form of the FIGURE 9 embodiment cathode is one Wherein an additional chamber similar to lower chamber 96 is connected between support plate 95 and the top end walls of chambers 90 through 94 which are now also provided with apertures similar to apertures 28a through 28e. It can thus be appreciated that any desired connection and interconnection of multiple cathode chambers may be employed to obtain higher power electron beams. An array of cathode chambers may employ electrical insulation between certain of the chambers and also employ interchamber bias as illustrated in FIGURES 4 and 5, if desired, to achieve particular operating conditions for the cathode. Operation characteristics for any of the multi-stage hollow cathodes are similar to that illustrated in FIGURE 2.

From the foregoing description it can be appreciated that our invention attains the objectives set forth in that it makes available a multi-stage hollow cathode which can be employed in an apparatus for irradiating materials by means of an electron beam. The cathode is comprised of a plurality of chambers which may be interconnected in any desired series or series-parallel arrangement and is operable in a relatively low pressure ionizable gaseous medium and at a relatively low negative voltage to produce an electron beam of significant power magnitude. Since the electron beam is nonthermionically generated, the rapid deterioration associated with high vacuum cathodes in contaminated atmospheres is significantly "reduced with our cathode. Further, our use of a relatively low pressure gaseous medium avoids the problems associated with maintaining high vacuum in conventional electron beam generators. Control of the beam intensity or power is obtained by variation of the cathode-to-anode potential and, or, gas pressure. The self-focussing feature of the herein described gaseous beam type apparatus, under appropriate conditions of cathode voltage and gas pressure, also permits total or substantial elimination of the elaborate conventional beam focussing techniques employed in high vacuum, themionically emitting electrode electron beam generators. However, additional beam focussing means may be employed to provide an independent beam focus adjustment as dictated by the particular electron beam irradiation application. Also, the apparatus may be used for irradiating materials in controlled environments including gaseous media different from the gas maintained in the vicinity of the cathode by dividing the housing into two or more enclosures, one for the cathode and another for the material being processed. The lower voltages employed eliminate, or very substantially reduce the generation of X-rays and thus no X-ray shielding is required. The lower voltages also permit use of more simplified electrical insulator elements to provide a more compact and less expensive electron beam irradiation apparatus, and more simplified, inexpensive power supply therefor.

Having described five embodiments of an improved apparatus for generating an electron beam at relatively low voltages in a relatively low pressure gaseous medium, it is believed obvious that modification and variation of our invention is possible in the light of the above teachings. Thus, alternating current power or any combination of direct and alternating current power may be applied to the cathode structure to obtain a controlled pulsating electron beam. Further, the multi-stage cathode structure and chambers therein may be of various forms, the criterion being that each chamber is provided with at least one aperture for interconnecting the plasma generated therein with a plasma in an adjacent chamber. Finally, gases other than argon and helium may be employed to develop the plasma bodies, hydrogen and nitrogen being further examples of the more suitable type. It is, therefore, to be understood that changes may be made in the particular embodiments as described which are within the full intended scope of the invention as defined by the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an improved electron beam irradiation apparatus comprising an enclosure containing a relatively low pressure ionizable gaseous medium, a hollow cathode positioned within said enclosure and operating circuit means effective for operating said cathode at a potential sufliciently negative relative to an anode in the operating cir cuit to initiate and maintain plasma within the hollow cathode by the interaction of the gaseous medium and the negative cathode-to-anode potential thereby to eflfect a nonthermionic electron beam mode of operation of said cathode, the improvement comprising said cathode comprising a hollow, electrically conduc= tive cathode structure having a beam exit aperture, and

means for partitioning said cathode structure into at least two interconnected cathode chambers each di-= mensioned to be capable of maintaining a body of ion= ized gas plasma therein in a desired mode of opera= tion, said plasma bodies being interconnected through said partitioning means whereby an electron beam issues from the plasma and is emitted through the e m pe t re.

2. In the apparatus set forth in claim 1 wherein said partitioning means comprises an electrically conductive member provided with a second aperture.

3. In the apparatus set forth in claim 1 wherein said cathode structure has an electrically conductive side wall characterized by a number of small openings therethrough.

4. In the apparatus set forth in claim 1 wherein said cathode structure has an electrically conductive nonperforated side Wall.

5. In the apparatus set forth in claim 2 wherein said partitioning means is characterized by a number of small openings therethrough.

6. In the apparatus set forth in claim 2 wherein said partitioning means has a nonperforated surface except for said second aperture.

7. In the apparatus set forth in claim 1 wherein said partitioning means comprises a pair of spaced apart electrically conductive members each provided with an aperture for interconnecting the plasma bodies, and

the operating circuit means includes a low voltage means for applying a relatively low voltage to said cathode structure, said low voltage means being in communication with a first of said at least two cathode chambers.

8. In the apparatus set forth in claim 7 and further comprising an electrically insulating member connected between the pair of electrically conductive members of said partitioning means to provide the spaced apart relationship thereof, said electrically insulating member provided with an aperture in alignment with said partitioning member apertures.

9. In the apparatus set forth in claim 1 and further comprising means for maintaining a selected potential difference between said cathode chambers during the desired mode of cathode operation.

10. In the apparatus set forth in claim 9 wherein said potential means comprises resistive means interconnecting said cathode chambers to provide a bias potential difference therebetween during the desired mode of cathode operation.

11. In the apparatus set forth in claim 9 wherein said potential means comprises means for applying a selected bias voltage to selected ones of said cathode chambers.

12. In the apparatus set forth in claim 3 and further comprising a ceramic sheath constructed of an electrically insulating' high secondary electron emission material closely fitted to the outer surface of the cathode.

13. In the apparatus set forth in claim 1 wherein said partitioning means comprises a plurality of electrically conductive members each provided with at least one aperture whereby an array of cathode chambers maybe coupled in series or series-parallel rela-= tionship to produce a relatively high power electron beam at relatively low voltages at which X-ray generation. is completely eliminated. 

